Nano-soldering to single atomic layer

ABSTRACT

A simple technique to solder submicron sized, ohmic contacts to nanostructures has been disclosed. The technique has several advantages over standard electron beam lithography methods, which are complex, costly, and can contaminate samples. To demonstrate the soldering technique graphene, a single atomic layer of carbon, has been contacted, and low- and high-field electronic transport properties have been measured.

STATEMENT OF GOVERNMENTAL SUPPORT

The invention described and claimed herein was made in part utilizingfunds supplied by the U.S. Department of Energy under Contract No.DE-AC02-05CH11231 and in part utilizing funds supplied by the NationalScience Foundation. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

This invention relates generally to joining of small structures, and,more specifically, to making electrical contacts to nanostructures.

The conventional method used to contact nanostructures electrically iselectron beam lithography. While having good resolution, the procedureis complex, expensive, and time-consuming. Moreover, the polymer resistsand solvents used in the process leave residues that often contaminatethe sample or device. As a result, the major contribution to the deviceresistance is not from the sample itself, but from the contact. Whilelithography-free contacting techniques have been developed, such as withthe use of shadow masks, they have their drawbacks. It would be usefulto have a method to contact nanostructures electrically that is simple,robust, inexpensive, suitable for large-scale manufacturing, and thatdoes not introduce contamination.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and others will be readily appreciated by theskilled artisan from the following description of illustrativeembodiments when read in conjunction with the accompanying drawings.

FIG. 1 is a schematic diagram of a nano-soldering system used to contactnanostructures. A scanning electron microscope image in the upper leftof the figure shows an indium solder spike ending in a 50 nm radius tip(scale bar, 1 μm). An optical microscope image in the upper right showsa contacted graphene device (scale bar, 10 μm).

FIG. 2 is a graph showing the current-voltage I-V characteristic of thesolder-contacted graphene device shown in the optical image in the inset(source-drain voltage V_(sd) in the range ±10 V; scale bar, 10 μm). Thedashed line is a linear fit, indicating a high-bias resistance of 5.7kΩ. The plot in the inset is a low-bias curve of same device, with aresistance of 6.0 kΩ. The back-gate voltage V_(bg)=0 V.

FIG. 3 a shows plots of as-measured two-terminal conductance G′ (V_(bg))for four solder-contacted graphene devices. The dashed black-lines havebeen fitted to the data with a modified Drude model as described below.FIG. 3 b shows plots of sheet conductivities for the same devicesobtained by subtracting the contact resistance as determined from thefits and accounting for the different device aspect ratios.

DETAILED DESCRIPTION

The preferred embodiments are illustrated in the context of makingelectrical contacts to nanostructures. The skilled artisan will readilyappreciate, however, that the materials and methods disclosed hereinwill have application in a number of other contexts where joining ofsmall structures is desirable, particularly where low cost andcontamination-free joining are important.

In one embodiment of the invention, a method of joining nanostructuresuses a miniaturized soldering technique. Surprisingly, with thistechnique sub-micron sized, ohmic contacts to nanostructures of evensingle-atom thickness can be made. The technique is simple, inexpensive,rapid, and entirely avoids sample contamination.

FIG. 1 is a schematic diagram that shows a nano-soldering system 100,according to an embodiment of the invention. Components of the systeminclude an optical microscope 102, a micromanipulator 104, and a heatedsample holder 106 on a microscope stage 108. The micromanipulator 104and the microscope stage 108 may each have an xyz stage. A scanningelectron microscope image in the upper left shows an indium solder spikeending in a 50 nm radius tip (scale bar, 1 μm). An optical microscopeimage (scale bar, 10 μm) in the upper right shows a graphene sheet (greyregion) with two indium contacts (bright spike regions).

In one embodiment of the invention the sample to be contacted is placedon a substrate on the sample holder along with a small bead of indium.In one arrangement, the temperature of the sample holder is raised toabout 170° C., twenty degrees above the melting point of indium. Inother arrangements, the temperature of the sample holder can be raisedto between about 150 and 200° C. A tungsten tip at room temperature isinserted into the molten indium bead using the microscope XYZtranslation stage and the micromanipulator XYZ translation stage, and aspike of solder is pulled out slowly. Submicron spike tips (FIG. 1, SEMimage) are possible with careful adjustment of the temperature andpullout rate. The sample and spike tip are then successively positionedand aligned under the microscope using both XYZ stages. The microscopestage is then raised quickly, fusing the solder spike onto the sampleand substrate as they come into contact. Once all contacts are made, thesample heater is turned off and the contacts solidify as shown in theoptical image in the upper right in FIG. 1.

In other arrangements, other low-temperature melting point alloys ofindium and tin (Indalloy 1E, 4, 121, 182, 290) are used to makecontacts. Indium can be especially useful because of its good adhesionto silicon oxide, a common substrate or substrate surface layer. Anymetal or alloy whose melting point is significantly lower than thelowest temperature at which the nanostructures or substrate to becontacted begin to undergo an undesirable change (e.g., oxidation,melting, decomposition) can be used in the embodiments of the invention.In one example, carbon nanostructures, which undergo significantoxidation in air only above 350° C., can be paired with solders thathave eutectic or melting points in the range 118-280° C.

With conventional (macro) soldering, flux and inert or forming gases areused to ensure good bonding. Flux and flux-bearing solders can beundesirable for making electrical contact at the nanoscale, as they canintroduce contaminant residues onto the samples. Surprisingly, it hasbeen found that excellent bonds can be formed using the nano-solderingtechniques disclosed herein without flux and/or inert or forming gases.It may be that because the tungsten tip is at room temperature when itdraws out the solder spike, oxidation of the solder spike is minimal.Sample oxidation is also negligible for carbon nanostructures, sinceoxidation in air is significant only above 350° C., and solders witheutectic or melting points in the range 118-280° C. are used.Nano-solder contacts have been found to be extremely reliable androbust.

Graphene (single sheets of graphite) has garnered much attention due toits interesting physics and its promise for a variety of applications.In one exemplary embodiment, graphene sheets are extracted bymicromechanical exfoliation. Single graphene sheets are identifiedoptically by contrast analysis and by the shape of the D peak in Ramanspectra made from the samples. Once a graphene sheet is isolated,contacts can be nano-soldered to produce a working device within minutesusing the method described herein. A typical optical image of anano-solder contacted graphene device with two terminals is shown in theupper right corner of FIG. 1. In one exemplary embodiment graphenesamples are usually about 10-20 μm in size with contact separations assmall as several microns.

FIG. 2 is a plot of the source-drain current I_(sd) of a nano-solderedgraphene device (shown in the optical image in the inset; scale bar is10 μm) measured as a function of source-drain voltage V_(sd) in therange ±10 V at room temperature in ambient conditions. The I-Vcharacteristic is linear even up to high source-drain voltages. Theresistance of the device is 5.7 kΩ, as determined from a linear fit tothe data (shown by the dashed line). The other inset, an I-V curve ofthe same device taken in the range V_(sd)=±10 mV, has a low-biasresistance of 6.0 kΩ, nominally different from the high-bias resistance.The back-gate voltage V_(bg) is 0 V.

Taking device geometry into account, a lower bound on the currentcarrying capacity of graphene in air on a silicon oxide substrate can beplaced at about 390 A/m sheet or 120 MA/cm² bulk assuming a graphenethickness of 3.35 Å, the graphite interlayer spacing. This bulk currentcarrying capacity, more than one hundred times that of a superconductor,is comparable to that of multi-wall carbon nanotubes, roughly 10⁹ A/cm².In vacuum (10⁻⁵ mbar), current densities as high as 500 A/m have beenobserved without device failure. Assuming zero contact resistance anddiffusive transport, the power density of the device in FIG. 2 is 16kW/cm², more than two orders of magnitude larger than current processorheat flux. With such high current carrying capacities and powerdensities, it is possible to construct graphene electronic devices thatdo not have heat dissipation problems.

FIG. 3 a shows plots of two-terminal conductances G′ for four soldereddevices measured as a function of back-gate voltage V_(bg) atroom-temperature in vacuum. The dashed black-lines are fits to the datawith a modified Drude model as described below. All graphene samples,identically prepared and nano-solder contacted without any annealing orprocessing, are remarkable in that their Dirac points V_(D) (thelocation of the conduction minimum) are within five volts of V_(bg)=0 V.This is in contrast to electron-beam lithographed devices, where V_(D)in the tens of volts is common. The clean, solder-contacted samples,without lithography residues to charge the sample and shift the Diracpoint away from zero, are at least neutral, if not undoped. This is thefirst indication of how processing parameters influence devicecharacteristics.

Without wishing to be bound to any particular theory, this analysis isoffered as one possibility for understand the surprising resultsdescribed herein. Although these measurements have been made using onlytwo terminals, we can estimate the effective, or device mobility, theminimum conductivity, and the contact resistance with a simple model. Werelate the Drude equation, σ=enμ with σ the conductivity and n thecarrier density, to the conductance using σ=GL/W. The experimentallymeasured conductance G′ includes the contact resistance R_(c) via1/G′=R_(c)+1/G. For graphene in a standard transistor geometry, thecarrier density depends on the back-gate voltage asn=c′|V_(bg)−V_(D)|/e, where the specific capacitance c′ for a 300 nmsilicon oxide gate thickness is 115 aF/μm². Finally, we add aphenomenological parameter σ_(D) to account for the non-zero minimumconductance and allow differing electron and hole mobilities μ_(e),μ_(h) to obtain

$\begin{matrix}{\frac{1}{G^{\prime}} = {R_{c} + \frac{L/W}{{{c\;}^{\prime}\mu_{e,h}{{V_{bg} - V_{D}}}} + \sigma_{D}}}} & (1)\end{matrix}$

using μ_(e) for back-gate voltage V_(bg)≧V_(D) and μ_(h) in the rangeV_(bg)<V_(D). The aspect ratio L/W is determined from optical images ofthe devices. In general, this model overestimates the contactresistance, as any intrinsic sub-linearity in the conductance-gatevoltage curves contributes to R_(c). While the data can also be fit tomore fundamental theories, the simple model may suffice to characterizethe graphene devices.

Having extracted the contact resistance from the curve fitting in FIG. 3a (dashed black lines), the sheet conductivities for the same devices,σ=L/W (1/G′−R_(c)) is plotted as a function of V_(bg) in FIG. 3 b. Thesheet conductivities are obtained by subtracting the contact resistanceas determined from the fits and accounting for the different deviceaspect ratios.

The conductivity curves are relatively linear for almost all devices,indicating the fit is good. The plots show that the mobilities (slopesof the curves) and minimal conductivities are roughly the same for alldevices. The electron mobilities range from 4500-6200 cm²/V·s and holemobilities range from 3000-3600 cm²/V·s, showing much less variationthan in electron-beam lithography defined devices. The minimalconductivities are 210 μS, 230 μS, 300 μS, and 440 μS. The contactresistance per lead, measured for nine devices, varied from 190-1700Ω,with mean 680Ω and standard deviation 450Ω, comparable to the bestelectron-lithography fabricated devices.

In another embodiment of the invention, the nano-solder contact methodcan be used to contact nanotubes, nanowires, nanospheres, or otherlinear or non-linear nanostructures. In one example, scanning electronmicroscopy (SEM) can be used to locate multi-wall carbon nanotubesrelative to predefined optically visible markers. The nano-solderingtechnique, as described above, can then be used by positioning the leadsrelative to the markers. In another example, a piezo micromanipulatorcan be used inside the SEM itself, along with a heated sample stage, andthe contacts can be soldered in situ.

In yet other embodiments, the nano-solder technique can be used forwirebonding and shadow mask alignment. To wirebond a device that alreadyhas a lead, the device is placed on a heated sample stage, a fine wireis placed near the lead and a solder spike is deposited, over both thelead and the wire. When the heat is turned off, the solidified spikefixes the wire to the substrate and provides electrical contact to thelead. The wirebonding and sample soldering can also be performed in asingle step, with the solder spike both contacting the sample and fixingthe wire. To align shadow masks, a similar process is used. The mask isplaced on top of the substrate, over the sample, and soldered at thecorners. The micromanipulator is then used to push the mask intoalignment, and the heater turned off to fix the mask.

Solder contacts are a simple, efficient means of producing functionalnanostructure devices, from graphene, nanotubes, or other materials. Notonly are the contacts ohmic, but the resultant devices are clean and thedevice characteristics consistent. The contacts, capable of sustaininglarge currents without failure, also allow investigation of high-biaselectronic transport properties.

This invention has been described herein in considerable detail toprovide those skilled in the art with information relevant to apply thenovel principles and to construct and use such specialized components asare required. However, it is to be understood that the invention can becarried out by different equipment, materials and devices, and thatvarious modifications, both as to the equipment and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

1. A method of making contact to nanostructures, comprising the stepsof: providing a melted solder bead; inserting a metal tip into thesolder bead; pulling the metal tip out from the solder bead to form asubmicron solder spike on the tip; providing a nanostructure on a heatedholder; contacting the nanostructure with the solder spike to form acontact; and allowing the heated holder to cool so that the contact cansolidify.
 2. The method of claim 1 wherein the nanostructure is on asubstrate and the contacting is between the nanostructure and thesubstrate.
 3. The method of claim 1 wherein the nanostructure is agraphene sheet.
 4. The method of claim 1 wherein the nanostructure is alinear nanostructure.
 5. The method of claim 1 wherein the nanostructureis a non-linear nanostructure.
 6. The method of claim 1 wherein themelted solder bead is on the heated holder.
 7. The method of claim 1wherein the metal tip is at room temperature before the inserting step.8. The method of claim 1 wherein the solder comprises indium.
 9. Themethod of claim 8 wherein the solder further comprises tin.
 10. Themethod of claim 9 wherein the heated holder has a temperature betweenabout 150° C. and 200° C.
 11. The method of claim 10 wherein the heatedholder has a temperature of about 170° C.
 12. The method of claim 1wherein the nanostructure comprises carbon and the heated holder has atemperature between about 110° C. and 300° C.
 13. The method of claim 12wherein the nanostructure comprises carbon and the heated holder has atemperature between about 118° C. and 280° C.
 14. The method of claim 1wherein the method is performed using an optical microscope.
 15. Themethod of claim 1 wherein the method is performed in a scanning electronmicroscope.
 16. A solder technique for wirebonding, comprising the stepsof: providing a melted solder bead; inserting a metal tip into thesolder bead; pulling the metal tip out from the solder bead to form asubmicron solder spike on the tip; providing a device with a lead on aheated holder; positioning a fine wire near the lead; contacting thelead and the fine wire with the solder spike to form a contact; andallowing the heated holder to cool so that the contact can solidify.